Aluminium and uncoated stainless fixings are not friends.

We are getting bubbling under the powder coating of a quality PTZ that has been installed outside for several years – what could be the issue here? The damage is rapidly increasing around bolts.

A: It sounds like corrosion caused by metals of different nobilities in contact with each other causing purified metal to go back to oxide form. The deterioration is a result of reaction with oxygen, sodium chloride and moisture in the environment. It’s increasingly common in cast aluminium PTZs and domes that are in contact with uncoated stainless steel fixings.

Oxidation of metals is an electrochemical reaction in which a natural battery is formed. The current that flows in this battery is central to an anodic process that sees metal disintegrate. The process can be accelerated when 2 metals with different nobilities are in contact with each other. During this process the metal with the least resistance to corrosion is always the one that corrodes first – in your case the aluminium PTZ housing – and it does so much faster than it would if corroding on its own.

Important to consider is that the anodic relationship applies to electronic security installations where cheap metals are used in screws and mountings in contact with high quality metal parts. As a result, you need to ensure housings are properly prepared and painted, or powder coated, with screws and bolts being marine grade stainless steel or galvanized steel coated before installation with rubber spacers installed wherever possible. There are coatings you can apply that will help parts resist corrosion. If you can’t find them at Bunnings try a Marine Chandlery like Whitworths.

The idea with any external metal device or component is to properly prepare and maintain it, painting and ensuring any nicks or scratches in the paintwork are retouched. You also need to ensure there’s no way moisture can build up on and around the metal conduit or its fittings. Fighting oxidisation is a life’s work – even the highest grades of stainless steel will eventually succumb, though much more slowly in the presence of an air gap.